The present invention relates to a multi-leaf collimator (MLC), such as the type commonly used in therapeutic linear accelerators.
In a conventional MLC, an array of parallel leaves move longitudinally alongside each other to define a irregular edge. A pair of such arrays are provided in a mutually opposing arrangement such that the pair together define a variable shape aperture which can then define a therapeutic dose of radiation. Diaphragms above or below the leaves can be moved into and out of the area covered by the array to reduce leakage between adjacent leaves.
Several factors mean that the construction and use of an MLC is not as simple. One such factor is that the tips of the leaves generally need to be profiled in order to give a constant and predictable penumbra, which means that an opposing pair cannot be brought into close enough contact to reduce leakage between the gap to an acceptable level.
The present invention therefore provides a collimator comprising first and second opposed arrays of individually longitudinally moveable elongate leaves, and a blocking element arranged transverse to the two arrays and moveable in that transverse direction, thereby to move into a gap between the tips of a leaf from the first array and a leaf from the second array.
The blocking element and the two arrays can be in the same plane, thereby reducing the overall height of the device.
As the tips of the leaves are usually profiled, it is preferred that the side edges of the blocking element are profiled to a complementary shape.
The side edges of the leaves are usually non-vertical in order to allow for the divergence of the radiation beam. Each leaf will have a different angle. Accordingly, it is preferred that the tip of the blocking element has a variable angle, in order to match the angle of whichever leaf is adjacent. This could be achieved, for example, by making at least the tip in a laminar fashion so that each lamination can slide to form a tip which is angled. Alternatively, a rotatable element could be incorporated into the tip.
It is also preferred that the blocking element has a flange which extends from a side thereof, positioned to extend adjacent over the tolerance gap between the blocking element and the leaf.
It is obviously preferred if there are two blocking elements, disposed on opposite sides.